I'm just a few months back in the audio scene after being very busy with family, career and other hobbies
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During the eighties and nineties I've worked in design, manufacturing and sales of high-end audio equipment. Most early CD players sounded outright awful. Harsh, flat soundstage and lifeless dynamics. Somewhere in the late eighties/early nineties they started to become acceptable. The CD players I liked most at that time used either the TDA1541(A, S1, S2) or the PCM64. And later the Ultra Analog D20400. All R-2R dacs. Then a new type of converter was introduced they called it Bitstream, MASH, delta/sigma. They all used a form of delta/sigma modulator to reconstruct the original waveform.
The technical difference is that a R-2R dac immediately switches between amplitude levels that represent the signal shape. A delta-sigma modulator produces a string of pulses with a certain density that 'add up' to the amplitude level that represents the signal shape. The reason for this development was the fact that its extremely difficult to produce a R-2R DAC with a true 16bit or higher resolution. This requires ultra precise calibration of The 'R' along the R-2R network. A delta/sigma modulator uses just one calibrated switch (or a few, like the Sabre does).
Thats the technical difference. What does this mean soundwise?
The best example for me has always been the development of the ARCAM products. One of my favorite players was the Delta 70.2, which used the TDA1541. It was succeeded by the 70.3 that used the SAA7350, the first bitstream DAC made by Philips (who also produced the TDA 1541). The 70.2 was really shining when it came to dynamics, impact and tonal balance. When the 70.3 came it showed better detail, refinement and depth of soundstage. But it was missing the impact and dynamics. For many years this difference has remained the same. But halfway the nineties chip designers and designers of audio equipment made some big advancements in their delta/sigma based designs. One of the most important ones was the fact that they realized that these dacs are much more sensitive to jitter and the quality of the output buffer/filter.
I doubt if I will ever find a DAC that can compete with the amazing Stax X1t which used the D20400. (I worked at the Dutch distributor of Stax at the time this DAC was introduced. I still remember every listening session we had with our demo unit!). But there are plenty CD players out there that will outperform a Marantz CD12/DA12 in terms of soundstage, low level detail and HF smoothness. And that will perform at leas on par when it comes to dynamics and impact.
A well implemented delta/sigma dac will outperform a mediocre R-2R design. How the best R-2R design will perform agains the same level of R-2R dac will be known soon...... As I assume that a large part of the head-fi Dedicated Source Components topics will be centering around the currently released Sabre based DACs from Audio-GD